A mechanical coolant pump is a coolant pump which is driven by the combustion engine, for example, by using a driving belt driving a driving wheel of the pump. As long as the combustion engine is cold, only a minimum coolant flow is needed. Therefore, mechanical coolant pumps are used which can vary the capacity of the coolant flow rate. As long as the combustion engine is cold, the flow rate is minimized, with the result that the combustion engine warming-up phase is shortened.
A mechanical coolant pump of the prior art which is able to vary the capacity of the coolant flow rate is disclosed in U.S. Pat. No. 4,752,183. The pump comprises a housing and a rotor shaft on which a pump wheel is mounted, whereby the pump wheel pumps the coolant radially outwardly. The pump wheel comprises a base disk and a separate valve disk. The base disk is provided with an axial inlet opening and is fixed on the rotor shaft. The valve disk is arranged separately on a disk shaft, whereby the disk shaft is incorporated into the rotor shaft and is axially movable so that the pump wheel can vary the coolant flow rate by varying the axial distance between the base disk and the valve disk, i.e., the radial outlet opening of the pump wheel. The rotor shaft on which the base disk is mounted is in the inlet area of the pump so that the rotor shaft is provided with a significant flow resistance for the coolant which is sucked axially by the pump wheel. This flow resistance causes turbulence in the coolant flow so that the energy consumption of the pump is high even when the pump is pumping with a minimal flow rate.